It has been previously shown by R. A. Jewell and G. F. Sykes in Preprints, Division of Organic Coatings and Plastics Chemistry, American Chemical Society, 36(2), 258 (1976); and by V. L. Bell, B. L. Stump, and H. Gager, J. Polym. Sci., Polym. Chem. Ed., 14, 2275 (1976) that methylene groups which connect two phenylene (benzenoid) rings having the structure shown below ##STR1## are readily attacked under high temperature oxidative conditions by a reaction mechanism involving radical species. Attack by methylene groups connecting two phenylene rings under high temperature oxidative conditions leads to radical species of the diphenylenemethylene type: ##STR2##
These new radicals on the polymers can react with each other or with other radical species to form stable covalent bonds between adjacent polymer chains, and so crosslinking them. ##STR3##
If oxygen is precluded from the thermal environment of the diphenylenemethylene-containing polymers, oxygen-containing radicals cannot be formed, and crosslinking is much less likely. Therefore, the thermooxidative process of Jewell et al involves the reaction of methylene groups with atmospheric oxygen at high temperatures (i.e., greater than 250.degree. C.). The Jewell et al process primarily results in oxidation of the methylene group to the carbonyl group. Some crosslinking occurs but the crosslinking reaction is secondary to the oxidation.
Aromatic polymers containing aliphatic groups other than methylene, such as ethylene (1,2-dimethylene), 1,3-trimethylene, 1,4-tetramethylene, etc., joining the aromatic moieties are quite susceptible to energy-caused cleavage, including irradiation. Therefore, irradiation may lead to degradation of the polymer chains as shown below ##STR4## Consequently, a drastic reduction of polymer molecular weight leads to an inevitable loss of desirable polymer properties.
Thus, methylene-containing polymers can be crosslinked by heating the polymers in air to 200.degree. C. or higher. However, that type of crosslinking is accompanied by polymer oxidation. Furthermore, oxygen-promoted crosslinking is limited to very thin films or fibers since oxygen cannot easily penetrate thick polymeric laminates or moldings, and the crosslinking can only take place on the surface of such articles.
Linear condensation polymers with predominantly aromatic structures generally have excellent mechanical properties and thermooxidative stability. However, the aromatic structures responsible for these attractive features also make these polymers difficult to process because of their relatively high softening and melting temperatures. In a study of the effects of aromatic isomerism on polymer properties by V. L. Bell et al, J. Polym. Sci., Polym. Chem. Ed., 14, 2275 (1976), it was demonstrated that the glass transition temperatures (Tg) of aromatic polyimides could be decreased markedly by the substitution of meta-phenylene groups for para-phenylene groups. The simple substitution of meta,meta' (m,m') oriented aromatic diamines for the customary para,para' (p,p') isomers reduced the Tgs of the polyimides by as much as 100.degree. C., usually with little or no sacrifice in thermooxidative or mechanical properties.
One of the drawbacks to the use of aromatic thermoplastic polyesters as an advanced composite matrix resin is the rather high thermal glass transition temperature (Tg) and polymer melt temperature (Tm) of these materials, as usually encountered with other aromatic homopolymers. Their high softening and melting temperatures are mainly due to the rigid structures resulting from the customary p,p' orientation of the diacid and diphenol monomers used for the synthesis of the aromatic polyester. Consequently, thermal processing and forming into useful articles is difficult.
It was fully expected that the Tgs of aromatic polyesters would be influenced by the same isomer effects as the polyimides. In fact, the 50/50 copolyester of isophthalic and terephthalic acid with bisphenol A (commercially available as ARDEL.RTM. D-100 from Union Carbide Corporation) is an engineering thermoplastic (Tg, 194.degree. C.; Tm, 260.degree. C.), described by G. Bier, Polymer, 15, 527 (1974), which exhibits good mechanical properties as shown by B. L. Dickinson in Modern Plastics Encyclopedia 1984-1985, McGraw Hill, New York, p. 45, 1984. However, ARDEL and most other easily thermoformed polyesters are highly sensitive to solvent induced stress crazing and cracking. ARDEL is soluble in many chlorinated solvents. Efforts were directed toward the twin goals of synthesizing aromatic polyesters with relatively low processing temperatures by the use of m,m'-phenylene linkages and finding a way to crosslink the polyesters after processing. Such a "cure" would not only diminish the thermoplastic nature of the polyesters by reducing creep at high temperatures, but would also moderate their solvent sensitivity.